(1) Field of the Invention
The present invention relates to an image forming apparatus that forms toner images on respective three photoreceptors by an electrophotographic process, and a power control device suitably used for such an image forming apparatus.
(2) Description of the Related Art
As an image forming apparatus that forms a full-color image by an electrophotographic method, a tandem-type color printer including four image forming units for forming toner images of respective Y (yellow), M (magenta), C (cyan), and K (black) colors is known. In such a tandem-type color printer, each image forming unit includes a photoreceptor drum.
In each image forming unit, a surface of the photoreceptor drum is uniformly charged by a charging device, and the charged surface of the photoreceptor drum is irradiated with a laser light to form an electrostatic latent image. The electrostatic latent image formed on the surface of the photoreceptor drum is developed with toner of a corresponding color Y, M, C, or K by a developing device included in the image forming unit. As a result, a toner image of the corresponding color Y, M, C, or K is formed on the surface (a photosensitive layer) of the photoreceptor drum.
One known example of a method for charging the photoreceptor drum is a method of using, as the charging device, a charging roller disposed to face the photoreceptor drum, and applying a composite voltage (field-production voltage) obtained by superimposing an AC voltage on a DC voltage to the charging roller, so that discharge is caused by a potential difference between the charging roller and the photoreceptor drum.
In this case, an amplitude (a potential difference) of the AC voltage included in the composite voltage is larger than a potential of the DC voltage. By applying such a composite voltage between the charging roller and the photoreceptor drum, the entire surface of the photoreceptor drum is almost uniformly charged to have a predetermined potential.
It is also known that, when the electrostatic latent image formed on the surface (photosensitive layer) of the photoreceptor drum is developed with toner in a developing device included in each process unit, the composite voltage obtained by superimposing the AC voltage on the DC voltage is applied, as a developing bias voltage, between the photoreceptor drum and a developing roller disposed to face the photoreceptor drum. In this case, the electrostatic latent image formed on the photoreceptor drum is developed, by an electric field produced between the developing roller and the photoreceptor drum, with toner that is conveyed on a surface of the developing roller.
In both cases, it is necessary to apply, for each process unit, a composite voltage appropriate for properties of the photosensitive layer of the photoreceptor drum, toner of each color, and the like, between the photoreceptor drum and the charging roller, or between the photoreceptor drum and the developing roller.
Patent Literature 1 (Japanese Patent Application Publication No. 5-197254) discloses an image forming apparatus that sequentially forms toner images of respective Y, M, C, and K colors on a surface of a single photoreceptor drum by using four developing devices each disposed to face the photoreceptor drum in a fixed manner. In the image forming apparatus disclosed in Patent Literature 1, a single developing power control device generates composite voltages, and the generated composite voltages are sequentially output to the respective four developing devices while performing high-speed switching by an electronic switch.
With such a structure, developing bias voltages applied to the respective four developing devices for Y, M, C, and K colors are generated by a single developing power control device. Compared to a case where four developing bias voltages are generated by respective four developing power control devices, the number of components is reduced, thereby leading to cost savings.
In the image forming apparatus disclosed in Patent Literature 1, in order to sequentially form four toner images on the single photoreceptor drum, composite voltages generated by the single developing power control device are applied to the respective developing rollers included in the four developing devices at different timings.
The structure disclosed in Patent Literature 1 applied to an image forming apparatus as disclosed in Patent Literature 1 including the single photoreceptor drum, however, is not applicable to an image forming apparatus including a plurality of photoreceptor drums as in the tandem-type printer described above. For example, in the tandem-type printer described above, toner images of respective Y, M, C, and K colors are formed on the respective photoreceptor drums included in the four process units almost at the same timing. It is therefore necessary to apply composite voltages between the photoreceptor drums and the respective charging rollers, or between the photoreceptor drums and the respective developing rollers at the same timing.
Furthermore, a photosensitive property and the like vary among photoreceptor drums included in the process units and toner of different colors has different properties. It is therefore also necessary to appropriately set, for each process unit, a composite voltage to be applied between the photoreceptor drum and the charging roller, or between the photoreceptor drum and the developing roller.
The structure disclosed in Patent Literature 1 in which composite voltages generated by the single developing power control device are applied to the respective four developing rollers at different timings is therefore not applicable to an image forming apparatus typified by a tandem-type printer.
Furthermore, in the structure disclosed in Patent Literature 1, an electronic switch having a transformer and the like is necessary to apply composite voltages generated by the single developing power control device to the respective four developing rollers at different timings. Such an electronic switch has a complex structure with a large number of components, and thus, even in a structure in which a single developing power control device is provided, the cost can be increased.
In an image forming apparatus including four process units typified by a tandem-type printer, it is necessary to generate an appropriate composite voltage for each of the four process units to charge the photoreceptor drum and to develop the electrostatic latent image formed on the photoreceptor drum.
The AC voltage included in the composite voltage is normally generated by an AC voltage generation circuit having a switching element and the like. If such an AC voltage generation circuit is provided for each process unit, the number of components can increase, thereby leading to increased cost.